Evohome + Opentherm + Viessmann Vitodens 100

[simonh]

I've seen other people using this combination on automated home. I've just setup the following system but it doesn't seem to be operating efficiently.

When a zone calls for heat the boiler fires up and seems to run at 80C. The return on this initially is > 20C drop. As the zone begins to warm a few degrees the boiler is still fired at 80C and the differential becomes 10C so the return flow is 70C or more! This outside condensing mode so not very efficient. The zone is my office which has 2 large convector heaters but there is a limit to how much heat they can disperse. Also, I personally find 80C radiators to be rather uncomfortable...

I followed the instruction on page 65 of the Vitodens installation book to limit the maximum heating input but this doesn't seem to make any difference when OpenTherm is in control it still fires to 80C. (http://www.viessmann-us.com/content/dam ... e/file.pdf)

Is there anyway on the evohome or the OpenTherm bridge to limit things?

-Simon

[Richard]

How long has evohome been in Simon?

There is a learning phase for OpenTherm control and this results in high flow temps in the initial period of 7-14 days. Past this the system will adjust accordingly and will be efficient with low modulation temps as you would expect.

I hope that helps.

Richard.

[simonh]

It's only been 24 hours. I'll just leave it to its onward see what happens.

[Richard]

Indeed, a little patience is needed sometimes with evohome. You have to bear in mind evohome needs to test its limits and is doing so with your boiler. You should notice over the next week it will start to range rate all by itself.

[stephencrown]

Just to update this thread for others, I have a very similar setup and basically have had to revert back to BDR91 Relay control.

Viessmann for some reason have locked out max ch flow tempmcontrol on the boiler as they expect the controller to configure this setting.


As the previous poster stated this results in a system of massive temps and massive overshoots.

The HR92 devices can not respond quick enough to compensate for the high flow temps, and a demand of 100% (which any zone 1-1.5 degrees under an HR92 will request), gets converted by the R8810 Opentherm bridge to the maximum ch flow temp available.

Honeywell defaults to 90 degrees C and expect the installer/end user to set an appropriate temp based on max heat loss in winter, and radiator sizing etc.

Because neither side (Viessmann boiler nor Honeywell controller/bridge) limit or have the abilaity to limit he max ch flow temp, this results in the boiler maxing out at the failsafe boiler limit of approx 81/82 degrees C.


I can categorically state with evidence and experience that the learning Richard mentions above does not deal with the problem stated, and that any zone below the setpoint when it comes on will force the boiler to full throttle and max temp where no boiler control/limit is possible and that no amount of learning helped and this was confirmed as normal behaviour by Honeywell tech support who were great and helped explain how it worked.


There is further info in the following forum/thread for further reading.


http://www.automatedhome.co.uk/vbulleti ... overshoots

Hope this helps someone avoid the same situation.

Also, it looks like Ideal boilers have a similar/same issue as described here:

http://www.automatedhome.co.uk/vbulleti ... -behaviour

[Richard]

Having read the threads, how have you ruled out the fact that you may not have bound the equipment correctly? I know every single variable in the evohome system that has taken 3 years to compile and no offence but we get Honeywell Connected Specialists get evohome wrong.

I have never known issues with Viessmann or Ideal like this before, but then I haven't spent my busy days looking for issues...

[stephencrown]

Hi

Both Viessmann and Honeywell have been actively (and in the case of Honeywell extensively) involved and confirmed the behavior is as described.

Honeywell provided modified (diagnostics,logging and telemetry based and focused) firmware on my Evohome controller to see what was happening and to try and diagnose the issue (the boiler was confirmed as the culprit by Honeywell)

Honeywell also sent out replacement Hardware to rule this out.

So I am sure it was configured correctly.

[Richard]

Hi Stephen,

I have come back on Automated Home Forum after seeing your post and it seems I was a little hasty when I posted the above comment.

After my own research, I can now see why these issues would arise (as I have stated on AH) and this is directly an issue with certain boilers who have not implemented the OpenTherm protocol with the view that the 'master' does not require all of the OpenTherm commands.

In the case of Viessmann, they are relying on an OpenTherm thermostat to set the max flow temp... This is not a requirement of the thermostat (master) and therefore this setting should be available on the boiler to be manually adjusted. Because the control set point can be changed to whatever the controls want, you are experiencing very hot flow temps when the evohome system calls for demand.

In my opinion, the boiler you have is not fit for purpose as they have ignored the fact OpenTherm thermostats don't need max set point command.

You have my sympathy with what you are experiencing and I hope you get this resolved.

Kind regards,

Richard

[stephencrown]

Cheers - I am stuck with the boiler until it's not economical to repair, so at present I'm trying/testing/using their weather compensation kit for Vitodens 1XX series boilers as a potential solution (the objective being reducing gas usage, therefore saving money ongoing and just as importantly to me reducing emissions).

I have the Weather Compensation kit as it was a last resort attempt to try and temper the max ch flow temp with OpenTherm, and I had a theory (that was proved false) that maybe the two could work together as some other manufacturers setups do.....so I had the kit setup anyway to test/use without OpenTherm once back in BDR91 control configuration.



It's also not great, for the simple reason there is only a single heat curve/gradient on the entry level Vitodens 1XX boilers. This can be shifted upwards/downwards which I thought may be enough control, but the current issue is that the curve/gradient out of the box (which cannot be changed) is too steep for the property (i.e. rads too cool on warmer days, rads to hot on colder days). It seems that only the next series up (Vitodens 2XX) boilers get proper gradient/slope control:

See page 4:
https://www.viessmann.co.uk/content/dam ... sation.pdf

As neither OpenTherm nor Weather Compensation is really ideal, my next plan (longer term) is looking at non-gas (i.e. heat pump) options for the future now and will plan around low temperature heating readiness retrofitting over the next few years (underfloor heating for an extension I am planning and into the existing hallway, extra and/or larger radiators for existing rooms, additional insulation etc.)


Hopefully this provides some additional info on OpenTherm alternatives when also not designed properly (i.e. Viessmann should not even bother releasing Weather Comp without slope/gradient controls for their lower end boilers!!!).

[flaviu.talos]

Hello everyone,

I saw that the manual mentioned in the first post is for 100w W1B1 and as far as i know this is an older model.(could be wrong).
I just bought an 100w B1HC model (latest) with wifi and touchscreen control and Evohome with R8810.

Does this issue apply to the latest model?Did anyone had the issue with the latest model.Can they confirm it doesn't work?Everyone is talking about 100w but we don't really know what model are we talking about.

Answer would be appreciated.

Thanks.